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Hydrothermal gasification of biomass model compounds (cellulose and lignin alkali) and model mixtures
J. of Supercritical Fluids 115 (2016) 79–85
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Hydrothermal gasification of biomass model compounds (cellulose and lignin alkali) and model mixtures Tülay Güngören Madeno˘glu ∗ , Mehmet Sa˘glam, Mithat Yüksel, Levent Ballice Ege University, Engineering Faculty, Department of Chemical Engineering, 35100 Bornova, I˙ zmir, Turkey
a r t i c l e
i n f o
Article history: Received 3 March 2016 Received in revised form 30 April 2016 Accepted 30 April 2016 Available online 2 May 2016 Keywords: Cellulose Lignin alkali Subcritical and supercritical water gasification Hydrogen Methane
a b s t r a c t Biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at sub- and super-critical water conditions in the absence and presence of alkali catalyst (K2 CO3 ). Hydrothermal gasification was performed in batch reactor at temperature range of 300–600 ◦ C and pressure range of 90–410 bar with a reaction time of 1 h. Product yields of gaseous, aqueous and residue were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to define the interaction between degradation products of model compounds of lignocellulosic biomass. In addition, gaseous and aqueous product compositions were identified to highlight prevailing reaction pathways during decomposition of model compounds. The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Supercritical water gasification (SCWG) emerges as an advantageous method that allowing conversion of biomass to hydrogen and/or methane without any pre-drying process. While valuable gas mixture produced during SCWG of biomass, undesired coke and tar production substantially reduced [1]. Studies on biomass gasification are mainly focused on its main constituents such as cellulose, hemicellulose and lignin. Hydrothermal gasification of glucose and cellulose as cellulose’ model, xylose, xylan, fructose and mannose as hemicellulose’ model and phenol, lignin alkali and lignin organosolv as lignin’ model compounds were performed to understand degradation mechanism of biomass. In addition, mixture of model compounds was prepared to represent biomass structure and to understand interaction between degradation products [2]. Alkali catalysts accelerate water-gas shift reaction yielding higher fraction of H2 and CO2 , as well as lower fraction of CO [3–5]. In addition, degradation temperature of biomass feed was lowered in the presence of alkali catalyst [6]. Metallic, alkali, carbon-based catalysts are the main types used in SCWG of biomass to enhance the gas yield and lower coke. Catalytic gasification of cellulose and pinewood to H2 in supercritical water was performed with Ni/CeO2 /Al2 O3 , dolomite, olivine and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Güngören Madeno˘glu). http://dx.doi.org/10.1016/j.supflu.2016.04.017 0896-8446/© 2016 Elsevier B.V. All rights reserved.
KOH as catalysts [7]. High alkalinity of catalyst (KOH) showed the best activity for H2 production from both precursors. Pooya et al. [8] were gasified glucose, cellulose, fructose, xylan, pulp, lignin and bark in supercritical water using nickel and ruthenium catalysts. Utilization of Raney nickel, Ru/C and Ru/Al2 O3 resulted in high methane yields. Gasification yields of cellulose and hemicellulose model compounds found higher than lignin and bark. As lignin is the most resistant component of biomass during gasification, it is important to improve its gasification for effective utilization of biomasses. The catalytic gasification of lignin alkali over Ru/C nanotubes in supercritical water showed an increased yield of hydrogen and methane [1]. Different lignin reagents (lignin sulfonate, lignin alkali, lignin hydrolytic, lignin organosolv and lignin organosolv acetate), cellulose, and their mixture were gasified with a nickel catalyst in supercritical water [2]. They found that different lignin reagents show different gasification characteristics and lower carbon gasification ratio was obtained with lignin alkali and lignin sulfonate. Nickel catalysts were deactivated by tarry products from the reaction between cellulose and lignin. The decomposition product distributions of 5- and 6-carbon sugars are not considerably different while quantitative variation was obtained due to difference in the structure of sugars and degradation rates [9–13]. Similar product distributions of hemicellulose model compounds that are xylose [14], fructose [15], mannose [11] were observed compared to cellulose model compounds that are cellulose [2,7,8] and glucose [10,16]. Because of these reason, only cellulose and lignin were selected as representative of
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biomass structure which are the main constituents of lignocellulosic biomass. Castello et al. [16] also investigated low temperature SCWG of cellulose and lignin that are model compounds as glucose and phenol to understand the fundamental phenomena involved in SCWG. In this study, sub- and super-critical water gasification of biomass model compounds (cellulose and lignin) and their mixtures were performed in the absence and presence of alkali catalyst (K2 CO3 ). The batch tests were conducted at temperature range of 300–600 ◦ C and pressure range of 90–410 bar with a reaction time of 1 h. Degradation of biomass was mimicked to identify condition at which higher H2 and CH4 yields can be reached. 2. Experimental 2.1. Samples Lignin alkali (Catalogue Number: 370959) and potassium carbonate, K2 CO3 , (Catalogue Number: 367877) were purchased from Sigma-Aldrich. Microcrystalline cellulose (Catalogue Number: 102331) was purchased from Merck. Composition of lignin alkali (Source: Norway spruce) is 48.4% C, 4.9% H, 0.1% N, 4% S and 42.6% O. 2.2. Reactor system Hydrothermal gasification studies were performed in a batch reactor with a volume of 100 mL. Batch reactor is made of stainless steel (SS316) and constructed to endure 50 MPa at 650 ◦ C. Detailed information on batch reactor system was given in a previous study [17]. 2.3. Experimental procedure Feed concentration was prepared as 0.45 M (1.2 g model compound (or model mixture)/15 mL water (or alkali catalyst solution)). Model mixtures (cellulose/lignin alkali) were prepared at different weight ratios that were 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to highlight the interaction between degradation products of model compounds of lignocellulosic biomass. Catalytic experiments were performed with alkali catalyst (K2 CO3 ) solution that was 10 wt.% of the feed mass (0.12 g K2 CO3 /15 mL water). Nitrogen gas was used to purge air in the reactor. Batch reactor was placed in electrical heater that can heat with a rate of 6 K min−1 . Reaction continued for 1 h after reaching reaction temperature. At the end of reaction time, batch reactor was rapidly cooled down by using fans and then placed in cooled water. The volume of gaseous products was measured by using gasometer and gaseous samples were collected in gas tight syringes to analyze in gas chromatography. Deviation in the volume of gaseous product was calculated as ±10%. The inside of the reactor was washed with water and then filtered to separate aqueous product and solid residue. The pH of aqueous products was lowered to 2 by addition of 1–2 drops of concentrated sulphuric acid which was required to inhibit ionization of organic acids. Aqueous products were stored in a refrigerator at 4 ◦ C. Detailed explanation on analysis of the gaseous, aqueous and solid products was given in Section 2.4. 2.4. Product analysis 2.4.1. Gaseous product analysis HP 7890A gas chromatography was used to analyze composition of the gaseous products (H2 , CO2 , CO and C1 –C4 hydrocarbons). Gas chromatography was equipped with 3 detectors (FID-TCD-TCD)
Table 1 Maximum pressure (bar) reached at different operating conditions for cellulose and lignin alkali. Cellulose
Lignin Alkali
T (◦ C)
No catalyst
K2 CO3
No catalyst
K2 CO3
300 400 500 600
90 210 300 360
95 190 305 345
100 210 290 345
95 205 330 380
Table 2 Maximum pressure (bar) reached at 600 ◦ C for cellulose/lignin alkali mixtures in the absence and presence of alkali catalyst. Cellulose/Lignin Alkali (wt./wt.)
No catalyst
K2 CO3
100/0 80/20 60/40 40/60 20/80 0/100
360 385 410 350 365 345
345 366 355 360 390 380
and serially connected 7 columns. Technical features and operating conditions of gas chromatography were given in previous work [18]. The standard deviation for the results of gas composition was calculated to be ±2%. 2.4.2. Aqueous product analysis Total organic carbon (TOC) content of aqueous phase was analyzed by a TOC analyzer (Shimadzu TOC-VCPH , Japan). Concentrations of the compounds (carboxylic acids, aldehydes, ketones, furfurals and phenols) in the aqueous products were analyzed by High Performance Liquid Chromatography (HPLC, Japan). All HPLC analyses were carried out using a Shimadzu LC-20A series liquid chromatography device equipped with an Inertsil ODS-3 column. The HPLC system consisted of a DGU-20AS degassing module, LC-20AT gradient pump, CTO-10ASVP chromatography oven and SPD-20 multi-wavelength ultraviolet detector. Analysis of calibration standards repeated for 5 times and calibration curves were prepared by plotting a linear regression of the average response factor versus analyte concentration. Carboxylic acids, phenols and furfurals were analyzed according to Method I, while aldehydes and ketones were analyzed by applying Method II [19]. The aldehydes and ketones were derivatized to their hydrozone forms by addition of 2,4-dinitrophenylhydrazine to aqueous samples. 2,4-dinitrophenylhydrazone forms of aldehydes and ketones were prepared as the same method described in the literature [20]. Unfortunately, “m- and p-cresols”, “methoxybenzene and 4-methoxyphenol”, “2-methoxyphenol and 3-methoxyphenol”, “acetone and propionaldehyde” cannot be separated from each other with present HPLC methods. 2.4.3. Residue analysis Solid sample module of a TOC analyzer (Shimadzu TOC-VCPH SSM-5000A, Japan) was used to determine the total organic carbon (TOC) content of the solid residue. 3. Results and discussion Sub- and super-critical water gasification of model compounds (cellulose and lignin alkali) were conducted at 300, 400, 500 and 600 ◦ C in the absence and presence of K2 CO3 . Maximum pressure (bar) reached at different operating conditions is given in Table 1. Hydrothermal gasification of model mixtures was conducted at four different ratios (wt./wt.) and at 600 ◦ C in the absence and presence of alkali catalyst (Table 2). Yields of products were calculated
T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85
by using following formulas on carbon (C) basis;
81
CH₄
H₂
CO₂
CO
C₂-C₄
100 Gas Composition ( mol/kg C in Feed )
mass of C in feed) × 100
Yield of residue (wt.%) = (mass of C in residue/ mass of C in feed) × 100
80
60
40
20
T(°C)
0
Yield of aqueous products (wt.%) = 100 − (Yield of gaseous
300 400 500 600
300 400 500 600
No Catalyst
products (wt.%) + Yield of residue (wt.%))
LIGNIN ALKALI
CELLULOSE
Yield of gaseous products (wt.%) = (mass of C in gaseous products/
300 400 500 600
K 2CO3
300 400 500 600
No Catalyst
K 2CO3
Fig. 1. Effects of temperature and alkali catalyst on gaseous product composition for hydrothermal gasification of cellulose and lignin alkali.
All experiments were performed in triplicate with an approximately 5% resulting precision of product yields. Reactor pressure was stabilized at desired value with a precision of ±0.5 MPa.
3.1. Product yields Effects of temperature and alkali catalyst on product yields (wt.%) of model compounds (cellulose and lignin alkali) are given in Table 3. Increase in temperature from 300 to 600 ◦ C yields higher gaseous product and lower aqueous-residue yields. As gasification rate increase with high reaction temperature and catalyst, increase of gaseous product yield is understandable. The highest gaseous product yield and lowest aqueous-residue yields were reached at 600 ◦ C in the presence of K2 CO3 . At this condition, cellulose was degraded to 81.2 wt.% gaseous products, while almost 61.4 wt.% of lignin alkali can be converted to gaseous products. K2 CO3 was found to be very effective in reduction of residue. Wang et. al reached 73.74% gasification efficiency for lignin alkali by using Ru/C nanotubes as catalyst [1]. Effects of model mixture (cellulose/lignin alkali) weight ratio and alkali catalyst on product yields (wt.%) for hydrothermal gasification at 600 ◦ C are given in Table 4. The influence of celluloselignin interaction is significant and this interaction affects the gaseous, aqueous and residue yields. Increase in lignin weight ratio of the mixture caused decrease in gaseous product yield and increase in aqueous product and residue yields. In the absence of catalyst, gaseous product yield decreased from 62.6 wt.% to 56.5 wt.% by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, gaseous product yield increased to 77.9 wt.% and 64.9 wt.% in the presence of K2 CO3 . As lignin is the most difficultly gasified component of biomass, increase in lignin alkali ratio caused decrease in gasification yields. Meanwhile, presence of alkali catalyst increased the gaseous product yield of the mixtures.
3.2. Gaseous products The gaseous product was mainly composed of hydrogen, carbon monoxide, carbon dioxide, and methane while ethane, ethene, propane, propene, and butane were identified as minors. Higher hydrocarbons were not detected. As the feed has different carbon content, gaseous product compositions were given in the unit of mol gas/kg C in feed. Effects of temperature and alkali catalysts on gaseous product composition for hydrothermal gasification of cellulose and lignin alkali were investigated in the absence and presence of alkali catalyst (K2 CO3 ) and given in Fig. 1. Yields of hydrogen, methane, carbon dioxide, and C2 –C4 increased and yield of carbon monoxide decreased with increasing temperature from 300 to 600 ◦ C for both of model compounds in the absence and presence of alkali catalyst. This situation can be explained by hydrogen and carbon dioxide yields increased with steam reforming reactions (Eq. (1)) and methane yield increased with methanation of carbon dioxide and carbon monoxide (Eqs. (2)–(3)). While carbon monoxide yield decreased as it was consumed in water-gas shift (WGS) reaction (Eq. (4)). Cellulose can hydrolyze to glucose and glucose reforming reaction can be written as; C6 H12 O6 ↔ 6CO + 6H2
(1)
Methane production was also improved by using alkali catalysts in supercritical conditions of water. Methanation reaction of CO; CO+3H2 ↔ CH4 + H2 O
(2)
Methanation reaction of CO2 ; CO2 +4H2 ↔ CH4 + 2H2 O
(3)
Table 3 Effects of temperature and alkali catalyst on product yields (wt.%) for hydrothermal gasification of cellulose and lignin alkali. Cellulose
Lignin Alkali
No catalyst ◦
K2 CO3
No catalyst
K2 CO3
T ( C)
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
300 400 500 600
21.1 34.2 60.0 66.2
31.3 22.1 4.3 2.7
47.6 43.7 35.7 31.1
29.9 40.6 66.7 81.2
32.6 27.6 9.0 3.7
37.5 31.8 24.3 15.1
12.0 23.5 44.5 54.9
26.6 19.8 8.6 6.7
61.4 56.7 46.9 38.4
18.0 27.6 54.3 61.4
34.2 26.3 11.8 6.9
47.8 46.1 33.9 31.7
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Table 4 Effects of cellulose/lignin alkali weight ratio and alkali catalyst on product yields (wt.%) for hydrothermal gasification at 600 ◦ C.
Cellulose/Lignin Alkali (wt./wt.)
Gaseous Aqueous Residue Gaseous Aqueous Residue
100/0 80/20 60/40 40/60 20/80 0/100
66.2 62.6 61.6 59.9 56.5 54.9
2.7 5.1 5.4 6.0 6.6 6.7
31.1 32.3 33.0 34.1 36.9 38.4
81.2 77.9 73.8 66.9 64.9 61.4
3.7 5.4 5.9 6.2 6.7 6.9
15.1 16.7 20.3 26.9 28.4 31.7
WGS reaction promoted to right side in supercritical conditions of water that leading a decrease in carbon monoxide and formation of carbon dioxide and hydrogen in the presence of alkali catalyst. Water-gas shift reaction; CO+H2 O ↔ CO2 + H2
H₂
CO₂
CO
C₂-C₄
K2CO3
NO CATALYST
K2 CO3
(4)
The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 . At this condition, gaseous products of cellulose consist of 30.7 mol H2 /kg C in cellulose and 22 mol CH4 /kg C in cellulose, while gaseous products of lignin alkali was 23.7 mol H2 /kg C in lignin alkali and 17.9 mol CH4 /kg C in lignin alkali. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification were investigated in the absence and presence of alkali catalyst (K2 CO3 ) at 600 ◦ C and given in Fig. 2. Yields of hydrogen, methane, carbon dioxide, carbon monoxide, and C2 –C4 decreased with increasing lignin ratio of the model mixture in the absence and presence of alkali catalyst. Alkali catalyst improved degradation of model mixture to gaseous products and yields of all analyzed gases increased for all mixture ratios. In the absence of catalyst, hydrogen yield decreased from 21.7 mol H2 /kg C in feed to 18.9 mol H2 /kg C in feed by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, hydrogen yield increased to 27.6 mol H2 /kg C in feed and 25.4 mol H2 /kg C in feed in the presence of K2 CO3 . In the absence of catalyst, methane yield decreased from 17.0 mol CH4 /kg C in feed to 15.3 mol CH4 /kg C in feed by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, methane yield increased to 20.5 mol CH4 /kg C in feed and 18.1 mol CH4 /kg C in feed in the presence of K2 CO3 . It can be concluded that lignin suppresses the formation of hydrogen and directly methane formation through methanation reaction where more hydrogen is required to shift reaction to right side. K2 CO3 found to be effective in cellulose and lignin degradation by yielding more hydrogen and methane. Inhibitory effect of phenol [16] and lignin [21] on H2 production or H2 consumption was observed in similar manner. 3.3. Aqueous products Cellulose hydrolyzes to glucose that isomerizes to fructose and mannose at low temperature of 250 ◦ C [22]. At subcritical conditions of water, these saccharides can dehydrate to furan compounds that are 5-methylfurfural, 5-hydroxymethylfurfural and furfural. Above the critical conditions of water, saccharides can hydrate to carboxylic acids via free radical reaction mechanism [23]. Lignin is a very complex compound that comprised of pcoumaryl, coniferyl or sinapyl alcohols that can hydrolyze to syringols, guaiacols, catechols, phenols and cresols. Further dehydration and dehydrogenation of these phenolic compounds cause coke formation that was higher at subcritical conditions of water. Above the critical conditions of water, phenolic compounds
Gas Composition ( mol/kg C in Feed )
No catalyst
CH₄ 100
80
60
40
20
0 100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Cellulose/Lignin Alkali (wt. / wt.)
Fig. 2. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification at 600 ◦ C.
degraded to gaseous products through hydration of intermadiates which can be phenols, carboxylic acids, aldehydes, ketones, and alcohols. Degradation products of model compounds (cellulose and lignin) that are representative of the main constituents of biomass are extensively studies at different operating conditions [6,8,21]. Mixture of model compounds was prepared at different weight ratios to mimic biomass composition. Interaction of degradation products during hydrothermal gasification was highlighted by identification of aqueous and gaseous product distributions. Since, carboxylic acids, furans, phenols, aldehydes, and ketones are the key compounds in aqueous products of biomass degradation [6], results were evaluated in this respect. Aqueous product compositions were given in the unit of g aqueous product/kg C in feed, as the feed has different carbon content. Main degradation products of model compounds and mixtures were carboxylic acids (especially acetic acid) and furfurals (especially 5-methyl furfural) at all operated conditions (Tables 5 and 6). Effects of temperature and alkali catalysts on aqueous product composition for hydrothermal gasification of cellulose and lignin alkali were investigated in the absence and presence of alkali catalyst (K2 CO3 ) and given in Table 5. Yield of each compound decreased with increasing temperature from 300 to 600 ◦ C, except formic acid and phenol. Formic acid and phenol yields increased up to 500 ◦ C, and almost dissipated at 600 ◦ C. Compared to aqueous product composition of cellulose with lignin alkali, especially yield of phenols, cresols, and aldehydes were found higher in lignin alkali. Lundquist and Ericsson [24] emphasized that lignin alkali was initially hydrolyzed to formaldehyde and phenol, and further to gaseous products. In addition, phenol and formaldehyde can react producing a cross-linking agent. Phenol-formaldehyde resin is an industrially produced polymerization product of phenol with formaldehyde. A study of Saisu et al. [25] show that phenolic structures can react with reactive sites in supercritical water leading to cross-linking reaction to produce high molecular weight (HMW) fragments. As a result, lignin is not only degraded to low-molecular weight compounds and also converted to HMWs that increases the char/tar. Simplified mechanism for conversion of cellulose/lignin to gaseous products through aqueous intermediates at hydrothermal conditions of water was given in Fig. 3. Yield of each compound in aqueous phase increased in the presence of alkali catalyst at all reaction temperatures. Same tendency for temperature and catalyst effects was confirmed with total organic carbon result that is shown in Fig. 4. Total organic carbon
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83
Table 5 Variation of aqueous product compositions (g aqueous product/kg C in feed) for hydrothermal gasification of cellulose and lignin alkali at different operating conditions. Cellulose
Lignin Alkali
No catalyst ◦
K2 CO3 ◦
◦
◦
◦
No catalyst ◦
◦
◦
◦
K2 CO3 ◦
◦
◦
Compound name
300 C
400 C
500 C
600 C
300 C
400 C
500 C
600 C
300 C
400 C
500 C
600 C
300 ◦ C
400 ◦ C
500 ◦ C
600 ◦ C
Hydroxyacetic acid Formic acid Acetic acid 5-Methyl furfural 1,4-Dioxane 5-Hydroxymethyl furfural Furfural 3-methyl-2-cyclopentene-1-on Acetone Phenol m- and p-cresols o-Cresol Formaldehyde Acetaldehyde Total phenols (colorimetric)
4.77 0.04 39.97 6.46 9.85 0.29 0.04 0.40 0.65 0.19 0.04 0.02 0.03 0.35 2.65
7.34 0.46 25.98 5.34 4.58 0.03 0.06 0.50 0.40 0.48 0.06 0.09 0.03 0.26 3.43
0.73 2.81 11.99 4.01 4.06 0.06 0.04 0.04 0.31 1.27 0.31 0.16 0.02 0.20 3.61
0.42 0.03 1.51 1.14 0.71 0.10 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.11 0.47
7.44 0.40 47.96 8.95 10.19 0.40 0.06 0.35 0.24 0.26 0.03 0.02 0.03 0.63 2.52
4.43 5.52 33.97 7.78 8.96 0.09 0.04 0.46 0.17 0.26 0.13 0.11 0.02 0.38 2.98
3.39 3.16 25.58 4.88 4.45 0.03 0.04 0.03 0.24 1.08 0.24 0.14 0.02 0.19 3.29
1.56 1.29 3.98 2.93 0.64 0.08 0.04 0.01 0.03 0.05 0.01 0.01 0.01 0.12 0.47
3.45 0.03 9.99 4.67 7.12 0.21 0.03 0.29 0.47 0.39 0.08 0.05 0.04 0.48 5.30
5.30 0.34 4.00 3.86 3.31 0.02 0.04 0.36 0.29 0.96 0.12 0.17 0.04 0.36 6.85
0.53 2.03 2.00 2.90 2.93 0.05 0.03 0.03 0.23 2.55 0.62 0.32 0.02 0.27 7.22
0.20 0.02 1.09 0.82 0.51 0.07 0.02 0.01 0.01 0.19 0.02 0.01 0.02 0.16 0.93
5.37 0.29 6.00 6.46 7.36 0.29 0.04 0.25 0.18 0.53 0.06 0.05 0.04 0.87 5.03
3.20 3.99 2.80 5.62 6.47 0.07 0.03 0.33 0.12 0.53 0.26 0.21 0.03 0.52 5.97
2.45 2.28 1.20 3.53 3.21 0.02 0.03 0.02 0.18 2.16 0.48 0.29 0.02 0.26 6.57
1.13 0.93 0.80 2.12 0.46 0.06 0.03 0.01 0.02 0.11 0.02 0.01 0.02 0.16 0.93
Fig. 3. Simplified mechanism for conversion of cellulose/lignin to gaseous products through aqueous intermediates at hydrothermal conditions of water.
2500
TOC of Aqueous Product (ppm)
CELLULOSE
LIGNIN ALKALI
2000
1500
1000
500
T(°C)
0 300 400 500 600
No Catalyst
300 400 500 600
K 2CO3
300 400 500 600
No Catalyst
300 400 500 600
K 2CO3
Fig. 4. Effects of temperature and alkali catalyst on TOC of aqueous products for hydrothermal gasification of cellulose and lignin alkali.
of aqueous products decreased with increasing temperature from 300 to 600 ◦ C for both of model compounds since aqueous products converted into gaseous products. As degradation of lignin is difficult compared to cellulose, total organic carbon of lignin’s aqueous product found higher. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification were investigated in the absence and presence of alkali catalyst (K2 CO3 ) at 600 ◦ C and given in Table 6. Yield of each compound decreased with increasing lignin alkali ratio in the model mixture
at constant temperature of 600 ◦ C, except phenol and acetaldehyde. As phenol and aldehyde are the main degradation products of lignin, phenol and acetaldehyde yields increased by increasing weight ratio of lignin alkali from 80/20 to 20/80. Moreover, carboxylic acids and furans decreased by increasing lignin alkali ratio in the mixture. Presence of alkali catalyst increased the aqueous product yield of each compound for all weight ratios. Alkali catalysts promote gasification rate and prevent char formation, so that aqueous product (intermediates) yield that is total organic carbon content increased (Fig. 5). It was known that added alkali was not only catalyst also a reactant that controls the formation of intermediate compounds in biomass gasification [26,27]. Muangrat et. al [28] suggested that sodium acetate could be the major component of the aqueous product in NaOH-catalysed hydrothermal gasification of biomass. Sodium carbonate and/or sodium bicarbonate can react with acetic acid to produce sodium acetate which can further react with water to produce methane. In similar manner, K2 CO3 can react with acetic acid, and further potassium acetate can hydrolyzed to methane. 2CH3 COOH + K2 CO3 → 2CH3 COOK + H2 O + CO2
(5)
CH3 COOK + H2 O → CH4 + KHCO3
(6)
These reactions explain how the methane content of the gaseous products increased in the presence of K2 CO3 . Moreover, increase in hydrogen and carbon dioxide yield in the presence of K2 CO3 can be explained by the enhancement of the water–gas shift reaction by intermediate formation of formates [27].
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Table 6 Variation of aqueous product compositions (g aqueous product/kg C in feed) for hydrothermal gasification of cellulose/ lignin alkali mixtures (wt./wt.) at 600 ◦ C in the absence and presence of alkali catalyst. No catalyst
K2 CO3
Compound name
100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Hydroxyacetic acid Formic acid Acetic acid 5-Methyl furfural 1,4-Dioxane 5-Hydroxymethyl furfural Furfural 3-methyl-2-cyclopentene-1-on Acetone Phenol m- and p-cresols o-Cresol Formaldehyde Acetaldehyde Total phenols (colorimetric)
0.42 0.03 1.51 1.14 0.71 0.10 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.11 0.47
0.35 0.03 1.36 1.03 0.64 0.09 0.03 0.01 0.01 0.11 0.01 0.01 0.01 0.12 0.53
0.31 0.02 1.28 0.97 0.60 0.09 0.03 0.01 0.01 0.12 0.01 0.01 0.01 0.13 0.62
0.27 0.02 1.20 0.90 0.56 0.08 0.03 0.01 0.01 0.14 0.01 0.01 0.01 0.13 0.71
0.23 0.02 1.12 0.84 0.52 0.08 0.02 0.01 0.01 0.16 0.01 0.01 0.01 0.14 0.80
0.20 0.02 1.09 0.82 0.51 0.07 0.02 0.01 0.01 0.19 0.02 0.01 0.02 0.16 0.93
1.56 1.29 3.98 2.93 0.64 0.08 0.04 0.01 0.03 0.05 0.01 0.01 0.01 0.12 0.47
1.41 1.16 3.27 2.65 0.58 0.07 0.04 0.01 0.03 0.06 0.01 0.01 0.01 0.12 0.52
1.31 1.08 2.48 2.45 0.53 0.07 0.04 0.01 0.03 0.07 0.01 0.01 0.01 0.13 0.63
1.26 1.04 2.07 2.36 0.52 0.06 0.04 0.01 0.03 0.08 0.01 0.01 0.01 0.14 0.70
1.14 0.94 1.27 2.14 0.47 0.06 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.15 0.81
1.13 0.93 0.80 2.12 0.46 0.06 0.03 0.01 0.02 0.11 0.02 0.01 0.02 0.16 0.93
No: 2008BIL017) and Ege University National Research Project Grants-BAP (Project No: 15MUH056). We would like to thank Mr. Gürsel Serin for the assistance during the experiments.
400
K2CO3
NO CATALYST
TOC of Aqueous Product (ppm)
350 300 250
References
200
[1] H. Wang, R. Miao, Y. Yang, Y. Qiao, Q. Zhang, C. Li, J. Huang, Study on the catalytic gasification of alkali lignin over Ru/C nanotubes in supercritical water, J. Fuel Chem. Technol. 43 (2015) 1195–1201. [2] Y. Takuya, O. Yoshito, M. Yukihiko, Gasification of biomass model compounds and real biomass in supercritical water, Biomass Bioenergy 26 (2004) 71–78. [3] A. Kruse, D. Meier, P. Rimbrecht, M. Schacht, Gasification of pyrocatechol in supercritical water in the presence of potassium hydroxide, Ind. Eng. Chem. Res. 39 (2000) 4842–4887. [4] M. Watanabe, H. Inomata, M. Osada, T. Sato, T. Adschiri, K. Arai, Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water, Fuel 82 (2003) 545–552. [5] A. Kruse, A. Gawlik, Biomass conversion in water at 330–410 ◦ C and 30–50 MPa identification of key compounds for indicating different chemical reaction pathways, Ind. Eng. Chem. Res. 42 (2003) 267–279. [6] Y. Matsumura, T. Minowa, B. Potic, S.R.A. Kersten, W. Prins, W.P.M. van Swaaij, et al., Biomass gasification in near- and supercritical water: status and prospects, Biomass Bioenergy 29 (2005) 269–292. [7] D. Ning, A. Ramin, K.D. Ajay, A.K. Janusz, Catalytic gasification of cellulose and pinewood to H2 in supercritical water, Fuel 118 (2014) 416–425. [8] A. Pooya, K. Sami, S. Friederike, A. Faraz, F. Ramin, Hydrogen production from cellulose, lignin, bark and model carbohydrates in supercritical water using nickel and ruthenium catalysts, Appl. Catal. B: Environ. 117–118 (2012) 330–338. [9] Y. Yu, X. Lou, H. Wu, Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods, Energy Fuels 22 (2008) 46–60. [10] T.G. Madeno˘glu, M. Sa˘glam, M. Yuksel, L. Ballice, Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose, J. Supercrit. Fluids 73 (2013) 151–160. [11] T.G. Madeno˘glu, N.Ü. Cengiz, M. Sa˘glam, M. Yuksel, L. Ballice, Catalytic gasification of mannose for hydrogen production in near- and super-critical water, J. Supercrit. Fluids 107 (2016) 153–162. [12] P.T. Williams, J. Onwudili, Composition of products from the supercritical water gasification of glucose: a model biomass compound, Ind. Eng. Chem. Res. 44 (2005) 8739–8749. [13] Y. Yu, X. Lou, H. Wu, Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods, Energy Fuels 22 (2008) 46–60. [14] K.G. Aaron, L.R. Gregory, Reaction rates for supercritical water gasification of xylose in a micro-tubular reactor, Chem. Eng. J. 163 (2010) 10–21. [15] S. Nanda, S.N. Reddy, H.N. Hunter, A.K. Dalai, J.A. Kozinski, Supercritical water gasification of fructose as a model compound for waste fruits and vegetables, J. Supercrit. Fluids 104 (2015) 112–121. [16] D. Castello, A. Kruse, L. Fiori, Low temperature supercritical water gasification of biomass constituents: glucose/phenol mixtures, Biomass Bioenergy 73 (2015) 84–94. [17] T. Gungoren, M. Saglam, M. Yuksel, H. Madenoglu, R. Isler, I.H. Metecan, et al., Near-critical and supercritical fluid extraction of industrial sewage sludge, Ind. Eng. Chem. Res. 46 (2007) 1051–1057. [18] T.G. Madeno˘glu, M. Sa˘glam, M. Yuksel, L. Ballice, Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose, J. Supercrit. Fluids 73 (2013) 151–160.
150 100 50 0 100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Cellulose/Lignin Alkali (wt. / wt.)
Fig. 5. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on TOC of aqueous products for hydrothermal gasification at 600 ◦ C.
4. Conclusion In the present work, biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at hydrothermal conditions in the absence and presence of alkali catalyst (K2 CO3 ). Gaseous, aqueous and residue yields were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.) to define the interaction between degradation products. The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 that promotes gasification rate and prevent char formation. It can be concluded that lignin suppresses the formation of hydrogen and directly methane formation. Main degradation aqueous products of model compounds and mixtures were carboxylic acids (especially acetic acid) and furfurals (especially 5-methyl furfural) at all operated conditions. Increase in lignin alkali ratio in the mixture caused a decrease in carboxylic acids and furans, and an increase in phenols and aldehydes. The results of model compound gasification will be useful for real biomass applications to predict the gaseous and aqueous products distributions. Acknowledgments We gratefully acknowledge the financial support provided by The Scientific and Technological Research Council of Turkey (TÜBI˙ TAK) (Project No: 106T748), Ege University Science and Technology Centre-Technology Transfer Office (EBILTEM-TTO) (Project
T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85 [19] T.G. Madeno˘glu, E. Yildir, M. Sa˘glam, M. Yuksel, L. Ballice, Improvement in hydrogen production from hard-shell nut residues by catalytic hydrothermal gasification, J. Supercrit. Fluids 95 (2014) 339–347. [20] R.F. Nascimento, J.C. Marques, B.S.L. Neto, D.D. Keukeleire, D.W. Franco, Qualitative and quantitative high-performance liquid chromatographic analysis of aldehydes in Brazilian sugar cane spirits and other distilled alcoholic beverages, J. Chromatogr. A 782 (1997) 13–23. [21] T. Yoshida, Y. Matsumura, Gasification of cellulose xylan, and lignin mixtures in supercritical water, Ind. Eng. Chem. Res. 40 (2001) 5469–5474. [22] D. Klingler, H. Vogel, Influence of process parameters on the hydrothermal decomposition and oxidation of glucose in sub- and supercritical water, J. Supercrit. Fluids 55 (2010) 259–270. [23] M. Watanabe, Y. Aizawa, T. Iida, C. Levy, T.M. Aida, H. Inomata, Glucose reactions within the heating period and the effect of heating rate on the reactions in hot compressed water, Carbohydr. Res. 340 (2005) 1931–1939. [24] K. Lundquist, L. Ericsson, Low-molecular weight lignin hydrolysis products, Appl. Polym. Symp. 28 (1976) 1393–1407. [25] M. Saisu, T. Sato, M. Watanabe, T. Adschiri, K. Arai, Conversion of lignin with supercritical water-phenol mixtures, Energy Fuels 17 (2003) 922–928.
85
[26] A. Kruse, H. Vogel, Heterogeneous catalysis in supercritical media: 2. Near-critical and supercritical water, Chem. Eng. Technol. 31 (2008) 1241–1245. [27] A. Sinag, A. Kruse, V. Schwarzkopf, Formation and degradation pathways of intermediate products formed during the hydropyrolysis of glucose as a model substance for wet biomass in a tubular reactor, Eng. Life Sci. 3 (2003) 469–473. [28] R. Muangrat, J.A. Onwudili, P.T. Williams, Alkali-promoted hydrothermal gasification of biomass food processing waste: a parametric study, Int. J. Hydrogen Energy 35 (2010) 7405–7415.
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Hydrothermal gasification of biomass model compounds (cellulose and lignin alkali) and model mixtures Tülay Güngören Madeno˘glu ∗ , Mehmet Sa˘glam, Mithat Yüksel, Levent Ballice Ege University, Engineering Faculty, Department of Chemical Engineering, 35100 Bornova, I˙ zmir, Turkey
a r t i c l e
i n f o
Article history: Received 3 March 2016 Received in revised form 30 April 2016 Accepted 30 April 2016 Available online 2 May 2016 Keywords: Cellulose Lignin alkali Subcritical and supercritical water gasification Hydrogen Methane
a b s t r a c t Biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at sub- and super-critical water conditions in the absence and presence of alkali catalyst (K2 CO3 ). Hydrothermal gasification was performed in batch reactor at temperature range of 300–600 ◦ C and pressure range of 90–410 bar with a reaction time of 1 h. Product yields of gaseous, aqueous and residue were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to define the interaction between degradation products of model compounds of lignocellulosic biomass. In addition, gaseous and aqueous product compositions were identified to highlight prevailing reaction pathways during decomposition of model compounds. The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Supercritical water gasification (SCWG) emerges as an advantageous method that allowing conversion of biomass to hydrogen and/or methane without any pre-drying process. While valuable gas mixture produced during SCWG of biomass, undesired coke and tar production substantially reduced [1]. Studies on biomass gasification are mainly focused on its main constituents such as cellulose, hemicellulose and lignin. Hydrothermal gasification of glucose and cellulose as cellulose’ model, xylose, xylan, fructose and mannose as hemicellulose’ model and phenol, lignin alkali and lignin organosolv as lignin’ model compounds were performed to understand degradation mechanism of biomass. In addition, mixture of model compounds was prepared to represent biomass structure and to understand interaction between degradation products [2]. Alkali catalysts accelerate water-gas shift reaction yielding higher fraction of H2 and CO2 , as well as lower fraction of CO [3–5]. In addition, degradation temperature of biomass feed was lowered in the presence of alkali catalyst [6]. Metallic, alkali, carbon-based catalysts are the main types used in SCWG of biomass to enhance the gas yield and lower coke. Catalytic gasification of cellulose and pinewood to H2 in supercritical water was performed with Ni/CeO2 /Al2 O3 , dolomite, olivine and
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (T. Güngören Madeno˘glu). http://dx.doi.org/10.1016/j.supflu.2016.04.017 0896-8446/© 2016 Elsevier B.V. All rights reserved.
KOH as catalysts [7]. High alkalinity of catalyst (KOH) showed the best activity for H2 production from both precursors. Pooya et al. [8] were gasified glucose, cellulose, fructose, xylan, pulp, lignin and bark in supercritical water using nickel and ruthenium catalysts. Utilization of Raney nickel, Ru/C and Ru/Al2 O3 resulted in high methane yields. Gasification yields of cellulose and hemicellulose model compounds found higher than lignin and bark. As lignin is the most resistant component of biomass during gasification, it is important to improve its gasification for effective utilization of biomasses. The catalytic gasification of lignin alkali over Ru/C nanotubes in supercritical water showed an increased yield of hydrogen and methane [1]. Different lignin reagents (lignin sulfonate, lignin alkali, lignin hydrolytic, lignin organosolv and lignin organosolv acetate), cellulose, and their mixture were gasified with a nickel catalyst in supercritical water [2]. They found that different lignin reagents show different gasification characteristics and lower carbon gasification ratio was obtained with lignin alkali and lignin sulfonate. Nickel catalysts were deactivated by tarry products from the reaction between cellulose and lignin. The decomposition product distributions of 5- and 6-carbon sugars are not considerably different while quantitative variation was obtained due to difference in the structure of sugars and degradation rates [9–13]. Similar product distributions of hemicellulose model compounds that are xylose [14], fructose [15], mannose [11] were observed compared to cellulose model compounds that are cellulose [2,7,8] and glucose [10,16]. Because of these reason, only cellulose and lignin were selected as representative of
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T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85
biomass structure which are the main constituents of lignocellulosic biomass. Castello et al. [16] also investigated low temperature SCWG of cellulose and lignin that are model compounds as glucose and phenol to understand the fundamental phenomena involved in SCWG. In this study, sub- and super-critical water gasification of biomass model compounds (cellulose and lignin) and their mixtures were performed in the absence and presence of alkali catalyst (K2 CO3 ). The batch tests were conducted at temperature range of 300–600 ◦ C and pressure range of 90–410 bar with a reaction time of 1 h. Degradation of biomass was mimicked to identify condition at which higher H2 and CH4 yields can be reached. 2. Experimental 2.1. Samples Lignin alkali (Catalogue Number: 370959) and potassium carbonate, K2 CO3 , (Catalogue Number: 367877) were purchased from Sigma-Aldrich. Microcrystalline cellulose (Catalogue Number: 102331) was purchased from Merck. Composition of lignin alkali (Source: Norway spruce) is 48.4% C, 4.9% H, 0.1% N, 4% S and 42.6% O. 2.2. Reactor system Hydrothermal gasification studies were performed in a batch reactor with a volume of 100 mL. Batch reactor is made of stainless steel (SS316) and constructed to endure 50 MPa at 650 ◦ C. Detailed information on batch reactor system was given in a previous study [17]. 2.3. Experimental procedure Feed concentration was prepared as 0.45 M (1.2 g model compound (or model mixture)/15 mL water (or alkali catalyst solution)). Model mixtures (cellulose/lignin alkali) were prepared at different weight ratios that were 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to highlight the interaction between degradation products of model compounds of lignocellulosic biomass. Catalytic experiments were performed with alkali catalyst (K2 CO3 ) solution that was 10 wt.% of the feed mass (0.12 g K2 CO3 /15 mL water). Nitrogen gas was used to purge air in the reactor. Batch reactor was placed in electrical heater that can heat with a rate of 6 K min−1 . Reaction continued for 1 h after reaching reaction temperature. At the end of reaction time, batch reactor was rapidly cooled down by using fans and then placed in cooled water. The volume of gaseous products was measured by using gasometer and gaseous samples were collected in gas tight syringes to analyze in gas chromatography. Deviation in the volume of gaseous product was calculated as ±10%. The inside of the reactor was washed with water and then filtered to separate aqueous product and solid residue. The pH of aqueous products was lowered to 2 by addition of 1–2 drops of concentrated sulphuric acid which was required to inhibit ionization of organic acids. Aqueous products were stored in a refrigerator at 4 ◦ C. Detailed explanation on analysis of the gaseous, aqueous and solid products was given in Section 2.4. 2.4. Product analysis 2.4.1. Gaseous product analysis HP 7890A gas chromatography was used to analyze composition of the gaseous products (H2 , CO2 , CO and C1 –C4 hydrocarbons). Gas chromatography was equipped with 3 detectors (FID-TCD-TCD)
Table 1 Maximum pressure (bar) reached at different operating conditions for cellulose and lignin alkali. Cellulose
Lignin Alkali
T (◦ C)
No catalyst
K2 CO3
No catalyst
K2 CO3
300 400 500 600
90 210 300 360
95 190 305 345
100 210 290 345
95 205 330 380
Table 2 Maximum pressure (bar) reached at 600 ◦ C for cellulose/lignin alkali mixtures in the absence and presence of alkali catalyst. Cellulose/Lignin Alkali (wt./wt.)
No catalyst
K2 CO3
100/0 80/20 60/40 40/60 20/80 0/100
360 385 410 350 365 345
345 366 355 360 390 380
and serially connected 7 columns. Technical features and operating conditions of gas chromatography were given in previous work [18]. The standard deviation for the results of gas composition was calculated to be ±2%. 2.4.2. Aqueous product analysis Total organic carbon (TOC) content of aqueous phase was analyzed by a TOC analyzer (Shimadzu TOC-VCPH , Japan). Concentrations of the compounds (carboxylic acids, aldehydes, ketones, furfurals and phenols) in the aqueous products were analyzed by High Performance Liquid Chromatography (HPLC, Japan). All HPLC analyses were carried out using a Shimadzu LC-20A series liquid chromatography device equipped with an Inertsil ODS-3 column. The HPLC system consisted of a DGU-20AS degassing module, LC-20AT gradient pump, CTO-10ASVP chromatography oven and SPD-20 multi-wavelength ultraviolet detector. Analysis of calibration standards repeated for 5 times and calibration curves were prepared by plotting a linear regression of the average response factor versus analyte concentration. Carboxylic acids, phenols and furfurals were analyzed according to Method I, while aldehydes and ketones were analyzed by applying Method II [19]. The aldehydes and ketones were derivatized to their hydrozone forms by addition of 2,4-dinitrophenylhydrazine to aqueous samples. 2,4-dinitrophenylhydrazone forms of aldehydes and ketones were prepared as the same method described in the literature [20]. Unfortunately, “m- and p-cresols”, “methoxybenzene and 4-methoxyphenol”, “2-methoxyphenol and 3-methoxyphenol”, “acetone and propionaldehyde” cannot be separated from each other with present HPLC methods. 2.4.3. Residue analysis Solid sample module of a TOC analyzer (Shimadzu TOC-VCPH SSM-5000A, Japan) was used to determine the total organic carbon (TOC) content of the solid residue. 3. Results and discussion Sub- and super-critical water gasification of model compounds (cellulose and lignin alkali) were conducted at 300, 400, 500 and 600 ◦ C in the absence and presence of K2 CO3 . Maximum pressure (bar) reached at different operating conditions is given in Table 1. Hydrothermal gasification of model mixtures was conducted at four different ratios (wt./wt.) and at 600 ◦ C in the absence and presence of alkali catalyst (Table 2). Yields of products were calculated
T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85
by using following formulas on carbon (C) basis;
81
CH₄
H₂
CO₂
CO
C₂-C₄
100 Gas Composition ( mol/kg C in Feed )
mass of C in feed) × 100
Yield of residue (wt.%) = (mass of C in residue/ mass of C in feed) × 100
80
60
40
20
T(°C)
0
Yield of aqueous products (wt.%) = 100 − (Yield of gaseous
300 400 500 600
300 400 500 600
No Catalyst
products (wt.%) + Yield of residue (wt.%))
LIGNIN ALKALI
CELLULOSE
Yield of gaseous products (wt.%) = (mass of C in gaseous products/
300 400 500 600
K 2CO3
300 400 500 600
No Catalyst
K 2CO3
Fig. 1. Effects of temperature and alkali catalyst on gaseous product composition for hydrothermal gasification of cellulose and lignin alkali.
All experiments were performed in triplicate with an approximately 5% resulting precision of product yields. Reactor pressure was stabilized at desired value with a precision of ±0.5 MPa.
3.1. Product yields Effects of temperature and alkali catalyst on product yields (wt.%) of model compounds (cellulose and lignin alkali) are given in Table 3. Increase in temperature from 300 to 600 ◦ C yields higher gaseous product and lower aqueous-residue yields. As gasification rate increase with high reaction temperature and catalyst, increase of gaseous product yield is understandable. The highest gaseous product yield and lowest aqueous-residue yields were reached at 600 ◦ C in the presence of K2 CO3 . At this condition, cellulose was degraded to 81.2 wt.% gaseous products, while almost 61.4 wt.% of lignin alkali can be converted to gaseous products. K2 CO3 was found to be very effective in reduction of residue. Wang et. al reached 73.74% gasification efficiency for lignin alkali by using Ru/C nanotubes as catalyst [1]. Effects of model mixture (cellulose/lignin alkali) weight ratio and alkali catalyst on product yields (wt.%) for hydrothermal gasification at 600 ◦ C are given in Table 4. The influence of celluloselignin interaction is significant and this interaction affects the gaseous, aqueous and residue yields. Increase in lignin weight ratio of the mixture caused decrease in gaseous product yield and increase in aqueous product and residue yields. In the absence of catalyst, gaseous product yield decreased from 62.6 wt.% to 56.5 wt.% by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, gaseous product yield increased to 77.9 wt.% and 64.9 wt.% in the presence of K2 CO3 . As lignin is the most difficultly gasified component of biomass, increase in lignin alkali ratio caused decrease in gasification yields. Meanwhile, presence of alkali catalyst increased the gaseous product yield of the mixtures.
3.2. Gaseous products The gaseous product was mainly composed of hydrogen, carbon monoxide, carbon dioxide, and methane while ethane, ethene, propane, propene, and butane were identified as minors. Higher hydrocarbons were not detected. As the feed has different carbon content, gaseous product compositions were given in the unit of mol gas/kg C in feed. Effects of temperature and alkali catalysts on gaseous product composition for hydrothermal gasification of cellulose and lignin alkali were investigated in the absence and presence of alkali catalyst (K2 CO3 ) and given in Fig. 1. Yields of hydrogen, methane, carbon dioxide, and C2 –C4 increased and yield of carbon monoxide decreased with increasing temperature from 300 to 600 ◦ C for both of model compounds in the absence and presence of alkali catalyst. This situation can be explained by hydrogen and carbon dioxide yields increased with steam reforming reactions (Eq. (1)) and methane yield increased with methanation of carbon dioxide and carbon monoxide (Eqs. (2)–(3)). While carbon monoxide yield decreased as it was consumed in water-gas shift (WGS) reaction (Eq. (4)). Cellulose can hydrolyze to glucose and glucose reforming reaction can be written as; C6 H12 O6 ↔ 6CO + 6H2
(1)
Methane production was also improved by using alkali catalysts in supercritical conditions of water. Methanation reaction of CO; CO+3H2 ↔ CH4 + H2 O
(2)
Methanation reaction of CO2 ; CO2 +4H2 ↔ CH4 + 2H2 O
(3)
Table 3 Effects of temperature and alkali catalyst on product yields (wt.%) for hydrothermal gasification of cellulose and lignin alkali. Cellulose
Lignin Alkali
No catalyst ◦
K2 CO3
No catalyst
K2 CO3
T ( C)
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
Gaseous
Aqueous
Residue
300 400 500 600
21.1 34.2 60.0 66.2
31.3 22.1 4.3 2.7
47.6 43.7 35.7 31.1
29.9 40.6 66.7 81.2
32.6 27.6 9.0 3.7
37.5 31.8 24.3 15.1
12.0 23.5 44.5 54.9
26.6 19.8 8.6 6.7
61.4 56.7 46.9 38.4
18.0 27.6 54.3 61.4
34.2 26.3 11.8 6.9
47.8 46.1 33.9 31.7
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Table 4 Effects of cellulose/lignin alkali weight ratio and alkali catalyst on product yields (wt.%) for hydrothermal gasification at 600 ◦ C.
Cellulose/Lignin Alkali (wt./wt.)
Gaseous Aqueous Residue Gaseous Aqueous Residue
100/0 80/20 60/40 40/60 20/80 0/100
66.2 62.6 61.6 59.9 56.5 54.9
2.7 5.1 5.4 6.0 6.6 6.7
31.1 32.3 33.0 34.1 36.9 38.4
81.2 77.9 73.8 66.9 64.9 61.4
3.7 5.4 5.9 6.2 6.7 6.9
15.1 16.7 20.3 26.9 28.4 31.7
WGS reaction promoted to right side in supercritical conditions of water that leading a decrease in carbon monoxide and formation of carbon dioxide and hydrogen in the presence of alkali catalyst. Water-gas shift reaction; CO+H2 O ↔ CO2 + H2
H₂
CO₂
CO
C₂-C₄
K2CO3
NO CATALYST
K2 CO3
(4)
The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 . At this condition, gaseous products of cellulose consist of 30.7 mol H2 /kg C in cellulose and 22 mol CH4 /kg C in cellulose, while gaseous products of lignin alkali was 23.7 mol H2 /kg C in lignin alkali and 17.9 mol CH4 /kg C in lignin alkali. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification were investigated in the absence and presence of alkali catalyst (K2 CO3 ) at 600 ◦ C and given in Fig. 2. Yields of hydrogen, methane, carbon dioxide, carbon monoxide, and C2 –C4 decreased with increasing lignin ratio of the model mixture in the absence and presence of alkali catalyst. Alkali catalyst improved degradation of model mixture to gaseous products and yields of all analyzed gases increased for all mixture ratios. In the absence of catalyst, hydrogen yield decreased from 21.7 mol H2 /kg C in feed to 18.9 mol H2 /kg C in feed by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, hydrogen yield increased to 27.6 mol H2 /kg C in feed and 25.4 mol H2 /kg C in feed in the presence of K2 CO3 . In the absence of catalyst, methane yield decreased from 17.0 mol CH4 /kg C in feed to 15.3 mol CH4 /kg C in feed by changing cellulose/lignin alkali ratio from 80/20 to 20/80 (wt./wt.), respectively. For the same cellulose/lignin alkali ratios, methane yield increased to 20.5 mol CH4 /kg C in feed and 18.1 mol CH4 /kg C in feed in the presence of K2 CO3 . It can be concluded that lignin suppresses the formation of hydrogen and directly methane formation through methanation reaction where more hydrogen is required to shift reaction to right side. K2 CO3 found to be effective in cellulose and lignin degradation by yielding more hydrogen and methane. Inhibitory effect of phenol [16] and lignin [21] on H2 production or H2 consumption was observed in similar manner. 3.3. Aqueous products Cellulose hydrolyzes to glucose that isomerizes to fructose and mannose at low temperature of 250 ◦ C [22]. At subcritical conditions of water, these saccharides can dehydrate to furan compounds that are 5-methylfurfural, 5-hydroxymethylfurfural and furfural. Above the critical conditions of water, saccharides can hydrate to carboxylic acids via free radical reaction mechanism [23]. Lignin is a very complex compound that comprised of pcoumaryl, coniferyl or sinapyl alcohols that can hydrolyze to syringols, guaiacols, catechols, phenols and cresols. Further dehydration and dehydrogenation of these phenolic compounds cause coke formation that was higher at subcritical conditions of water. Above the critical conditions of water, phenolic compounds
Gas Composition ( mol/kg C in Feed )
No catalyst
CH₄ 100
80
60
40
20
0 100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Cellulose/Lignin Alkali (wt. / wt.)
Fig. 2. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification at 600 ◦ C.
degraded to gaseous products through hydration of intermadiates which can be phenols, carboxylic acids, aldehydes, ketones, and alcohols. Degradation products of model compounds (cellulose and lignin) that are representative of the main constituents of biomass are extensively studies at different operating conditions [6,8,21]. Mixture of model compounds was prepared at different weight ratios to mimic biomass composition. Interaction of degradation products during hydrothermal gasification was highlighted by identification of aqueous and gaseous product distributions. Since, carboxylic acids, furans, phenols, aldehydes, and ketones are the key compounds in aqueous products of biomass degradation [6], results were evaluated in this respect. Aqueous product compositions were given in the unit of g aqueous product/kg C in feed, as the feed has different carbon content. Main degradation products of model compounds and mixtures were carboxylic acids (especially acetic acid) and furfurals (especially 5-methyl furfural) at all operated conditions (Tables 5 and 6). Effects of temperature and alkali catalysts on aqueous product composition for hydrothermal gasification of cellulose and lignin alkali were investigated in the absence and presence of alkali catalyst (K2 CO3 ) and given in Table 5. Yield of each compound decreased with increasing temperature from 300 to 600 ◦ C, except formic acid and phenol. Formic acid and phenol yields increased up to 500 ◦ C, and almost dissipated at 600 ◦ C. Compared to aqueous product composition of cellulose with lignin alkali, especially yield of phenols, cresols, and aldehydes were found higher in lignin alkali. Lundquist and Ericsson [24] emphasized that lignin alkali was initially hydrolyzed to formaldehyde and phenol, and further to gaseous products. In addition, phenol and formaldehyde can react producing a cross-linking agent. Phenol-formaldehyde resin is an industrially produced polymerization product of phenol with formaldehyde. A study of Saisu et al. [25] show that phenolic structures can react with reactive sites in supercritical water leading to cross-linking reaction to produce high molecular weight (HMW) fragments. As a result, lignin is not only degraded to low-molecular weight compounds and also converted to HMWs that increases the char/tar. Simplified mechanism for conversion of cellulose/lignin to gaseous products through aqueous intermediates at hydrothermal conditions of water was given in Fig. 3. Yield of each compound in aqueous phase increased in the presence of alkali catalyst at all reaction temperatures. Same tendency for temperature and catalyst effects was confirmed with total organic carbon result that is shown in Fig. 4. Total organic carbon
T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85
83
Table 5 Variation of aqueous product compositions (g aqueous product/kg C in feed) for hydrothermal gasification of cellulose and lignin alkali at different operating conditions. Cellulose
Lignin Alkali
No catalyst ◦
K2 CO3 ◦
◦
◦
◦
No catalyst ◦
◦
◦
◦
K2 CO3 ◦
◦
◦
Compound name
300 C
400 C
500 C
600 C
300 C
400 C
500 C
600 C
300 C
400 C
500 C
600 C
300 ◦ C
400 ◦ C
500 ◦ C
600 ◦ C
Hydroxyacetic acid Formic acid Acetic acid 5-Methyl furfural 1,4-Dioxane 5-Hydroxymethyl furfural Furfural 3-methyl-2-cyclopentene-1-on Acetone Phenol m- and p-cresols o-Cresol Formaldehyde Acetaldehyde Total phenols (colorimetric)
4.77 0.04 39.97 6.46 9.85 0.29 0.04 0.40 0.65 0.19 0.04 0.02 0.03 0.35 2.65
7.34 0.46 25.98 5.34 4.58 0.03 0.06 0.50 0.40 0.48 0.06 0.09 0.03 0.26 3.43
0.73 2.81 11.99 4.01 4.06 0.06 0.04 0.04 0.31 1.27 0.31 0.16 0.02 0.20 3.61
0.42 0.03 1.51 1.14 0.71 0.10 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.11 0.47
7.44 0.40 47.96 8.95 10.19 0.40 0.06 0.35 0.24 0.26 0.03 0.02 0.03 0.63 2.52
4.43 5.52 33.97 7.78 8.96 0.09 0.04 0.46 0.17 0.26 0.13 0.11 0.02 0.38 2.98
3.39 3.16 25.58 4.88 4.45 0.03 0.04 0.03 0.24 1.08 0.24 0.14 0.02 0.19 3.29
1.56 1.29 3.98 2.93 0.64 0.08 0.04 0.01 0.03 0.05 0.01 0.01 0.01 0.12 0.47
3.45 0.03 9.99 4.67 7.12 0.21 0.03 0.29 0.47 0.39 0.08 0.05 0.04 0.48 5.30
5.30 0.34 4.00 3.86 3.31 0.02 0.04 0.36 0.29 0.96 0.12 0.17 0.04 0.36 6.85
0.53 2.03 2.00 2.90 2.93 0.05 0.03 0.03 0.23 2.55 0.62 0.32 0.02 0.27 7.22
0.20 0.02 1.09 0.82 0.51 0.07 0.02 0.01 0.01 0.19 0.02 0.01 0.02 0.16 0.93
5.37 0.29 6.00 6.46 7.36 0.29 0.04 0.25 0.18 0.53 0.06 0.05 0.04 0.87 5.03
3.20 3.99 2.80 5.62 6.47 0.07 0.03 0.33 0.12 0.53 0.26 0.21 0.03 0.52 5.97
2.45 2.28 1.20 3.53 3.21 0.02 0.03 0.02 0.18 2.16 0.48 0.29 0.02 0.26 6.57
1.13 0.93 0.80 2.12 0.46 0.06 0.03 0.01 0.02 0.11 0.02 0.01 0.02 0.16 0.93
Fig. 3. Simplified mechanism for conversion of cellulose/lignin to gaseous products through aqueous intermediates at hydrothermal conditions of water.
2500
TOC of Aqueous Product (ppm)
CELLULOSE
LIGNIN ALKALI
2000
1500
1000
500
T(°C)
0 300 400 500 600
No Catalyst
300 400 500 600
K 2CO3
300 400 500 600
No Catalyst
300 400 500 600
K 2CO3
Fig. 4. Effects of temperature and alkali catalyst on TOC of aqueous products for hydrothermal gasification of cellulose and lignin alkali.
of aqueous products decreased with increasing temperature from 300 to 600 ◦ C for both of model compounds since aqueous products converted into gaseous products. As degradation of lignin is difficult compared to cellulose, total organic carbon of lignin’s aqueous product found higher. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on gaseous product composition for hydrothermal gasification were investigated in the absence and presence of alkali catalyst (K2 CO3 ) at 600 ◦ C and given in Table 6. Yield of each compound decreased with increasing lignin alkali ratio in the model mixture
at constant temperature of 600 ◦ C, except phenol and acetaldehyde. As phenol and aldehyde are the main degradation products of lignin, phenol and acetaldehyde yields increased by increasing weight ratio of lignin alkali from 80/20 to 20/80. Moreover, carboxylic acids and furans decreased by increasing lignin alkali ratio in the mixture. Presence of alkali catalyst increased the aqueous product yield of each compound for all weight ratios. Alkali catalysts promote gasification rate and prevent char formation, so that aqueous product (intermediates) yield that is total organic carbon content increased (Fig. 5). It was known that added alkali was not only catalyst also a reactant that controls the formation of intermediate compounds in biomass gasification [26,27]. Muangrat et. al [28] suggested that sodium acetate could be the major component of the aqueous product in NaOH-catalysed hydrothermal gasification of biomass. Sodium carbonate and/or sodium bicarbonate can react with acetic acid to produce sodium acetate which can further react with water to produce methane. In similar manner, K2 CO3 can react with acetic acid, and further potassium acetate can hydrolyzed to methane. 2CH3 COOH + K2 CO3 → 2CH3 COOK + H2 O + CO2
(5)
CH3 COOK + H2 O → CH4 + KHCO3
(6)
These reactions explain how the methane content of the gaseous products increased in the presence of K2 CO3 . Moreover, increase in hydrogen and carbon dioxide yield in the presence of K2 CO3 can be explained by the enhancement of the water–gas shift reaction by intermediate formation of formates [27].
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T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85
Table 6 Variation of aqueous product compositions (g aqueous product/kg C in feed) for hydrothermal gasification of cellulose/ lignin alkali mixtures (wt./wt.) at 600 ◦ C in the absence and presence of alkali catalyst. No catalyst
K2 CO3
Compound name
100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Hydroxyacetic acid Formic acid Acetic acid 5-Methyl furfural 1,4-Dioxane 5-Hydroxymethyl furfural Furfural 3-methyl-2-cyclopentene-1-on Acetone Phenol m- and p-cresols o-Cresol Formaldehyde Acetaldehyde Total phenols (colorimetric)
0.42 0.03 1.51 1.14 0.71 0.10 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.11 0.47
0.35 0.03 1.36 1.03 0.64 0.09 0.03 0.01 0.01 0.11 0.01 0.01 0.01 0.12 0.53
0.31 0.02 1.28 0.97 0.60 0.09 0.03 0.01 0.01 0.12 0.01 0.01 0.01 0.13 0.62
0.27 0.02 1.20 0.90 0.56 0.08 0.03 0.01 0.01 0.14 0.01 0.01 0.01 0.13 0.71
0.23 0.02 1.12 0.84 0.52 0.08 0.02 0.01 0.01 0.16 0.01 0.01 0.01 0.14 0.80
0.20 0.02 1.09 0.82 0.51 0.07 0.02 0.01 0.01 0.19 0.02 0.01 0.02 0.16 0.93
1.56 1.29 3.98 2.93 0.64 0.08 0.04 0.01 0.03 0.05 0.01 0.01 0.01 0.12 0.47
1.41 1.16 3.27 2.65 0.58 0.07 0.04 0.01 0.03 0.06 0.01 0.01 0.01 0.12 0.52
1.31 1.08 2.48 2.45 0.53 0.07 0.04 0.01 0.03 0.07 0.01 0.01 0.01 0.13 0.63
1.26 1.04 2.07 2.36 0.52 0.06 0.04 0.01 0.03 0.08 0.01 0.01 0.01 0.14 0.70
1.14 0.94 1.27 2.14 0.47 0.06 0.03 0.01 0.02 0.09 0.01 0.01 0.01 0.15 0.81
1.13 0.93 0.80 2.12 0.46 0.06 0.03 0.01 0.02 0.11 0.02 0.01 0.02 0.16 0.93
No: 2008BIL017) and Ege University National Research Project Grants-BAP (Project No: 15MUH056). We would like to thank Mr. Gürsel Serin for the assistance during the experiments.
400
K2CO3
NO CATALYST
TOC of Aqueous Product (ppm)
350 300 250
References
200
[1] H. Wang, R. Miao, Y. Yang, Y. Qiao, Q. Zhang, C. Li, J. Huang, Study on the catalytic gasification of alkali lignin over Ru/C nanotubes in supercritical water, J. Fuel Chem. Technol. 43 (2015) 1195–1201. [2] Y. Takuya, O. Yoshito, M. Yukihiko, Gasification of biomass model compounds and real biomass in supercritical water, Biomass Bioenergy 26 (2004) 71–78. [3] A. Kruse, D. Meier, P. Rimbrecht, M. Schacht, Gasification of pyrocatechol in supercritical water in the presence of potassium hydroxide, Ind. Eng. Chem. Res. 39 (2000) 4842–4887. [4] M. Watanabe, H. Inomata, M. Osada, T. Sato, T. Adschiri, K. Arai, Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water, Fuel 82 (2003) 545–552. [5] A. Kruse, A. Gawlik, Biomass conversion in water at 330–410 ◦ C and 30–50 MPa identification of key compounds for indicating different chemical reaction pathways, Ind. Eng. Chem. Res. 42 (2003) 267–279. [6] Y. Matsumura, T. Minowa, B. Potic, S.R.A. Kersten, W. Prins, W.P.M. van Swaaij, et al., Biomass gasification in near- and supercritical water: status and prospects, Biomass Bioenergy 29 (2005) 269–292. [7] D. Ning, A. Ramin, K.D. Ajay, A.K. Janusz, Catalytic gasification of cellulose and pinewood to H2 in supercritical water, Fuel 118 (2014) 416–425. [8] A. Pooya, K. Sami, S. Friederike, A. Faraz, F. Ramin, Hydrogen production from cellulose, lignin, bark and model carbohydrates in supercritical water using nickel and ruthenium catalysts, Appl. Catal. B: Environ. 117–118 (2012) 330–338. [9] Y. Yu, X. Lou, H. Wu, Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods, Energy Fuels 22 (2008) 46–60. [10] T.G. Madeno˘glu, M. Sa˘glam, M. Yuksel, L. Ballice, Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose, J. Supercrit. Fluids 73 (2013) 151–160. [11] T.G. Madeno˘glu, N.Ü. Cengiz, M. Sa˘glam, M. Yuksel, L. Ballice, Catalytic gasification of mannose for hydrogen production in near- and super-critical water, J. Supercrit. Fluids 107 (2016) 153–162. [12] P.T. Williams, J. Onwudili, Composition of products from the supercritical water gasification of glucose: a model biomass compound, Ind. Eng. Chem. Res. 44 (2005) 8739–8749. [13] Y. Yu, X. Lou, H. Wu, Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods, Energy Fuels 22 (2008) 46–60. [14] K.G. Aaron, L.R. Gregory, Reaction rates for supercritical water gasification of xylose in a micro-tubular reactor, Chem. Eng. J. 163 (2010) 10–21. [15] S. Nanda, S.N. Reddy, H.N. Hunter, A.K. Dalai, J.A. Kozinski, Supercritical water gasification of fructose as a model compound for waste fruits and vegetables, J. Supercrit. Fluids 104 (2015) 112–121. [16] D. Castello, A. Kruse, L. Fiori, Low temperature supercritical water gasification of biomass constituents: glucose/phenol mixtures, Biomass Bioenergy 73 (2015) 84–94. [17] T. Gungoren, M. Saglam, M. Yuksel, H. Madenoglu, R. Isler, I.H. Metecan, et al., Near-critical and supercritical fluid extraction of industrial sewage sludge, Ind. Eng. Chem. Res. 46 (2007) 1051–1057. [18] T.G. Madeno˘glu, M. Sa˘glam, M. Yuksel, L. Ballice, Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose, J. Supercrit. Fluids 73 (2013) 151–160.
150 100 50 0 100/0
80/20
60/40
40/60
20/80
0/100
100/0
80/20
60/40
40/60
20/80
0/100
Cellulose/Lignin Alkali (wt. / wt.)
Fig. 5. Effects of cellulose/lignin alkali weight ratio and alkali catalyst on TOC of aqueous products for hydrothermal gasification at 600 ◦ C.
4. Conclusion In the present work, biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at hydrothermal conditions in the absence and presence of alkali catalyst (K2 CO3 ). Gaseous, aqueous and residue yields were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.) to define the interaction between degradation products. The highest hydrogen and methane yields were reached at 600 ◦ C in the presence of K2 CO3 that promotes gasification rate and prevent char formation. It can be concluded that lignin suppresses the formation of hydrogen and directly methane formation. Main degradation aqueous products of model compounds and mixtures were carboxylic acids (especially acetic acid) and furfurals (especially 5-methyl furfural) at all operated conditions. Increase in lignin alkali ratio in the mixture caused a decrease in carboxylic acids and furans, and an increase in phenols and aldehydes. The results of model compound gasification will be useful for real biomass applications to predict the gaseous and aqueous products distributions. Acknowledgments We gratefully acknowledge the financial support provided by The Scientific and Technological Research Council of Turkey (TÜBI˙ TAK) (Project No: 106T748), Ege University Science and Technology Centre-Technology Transfer Office (EBILTEM-TTO) (Project
T. Güngören Madeno˘glu et al. / J. of Supercritical Fluids 115 (2016) 79–85 [19] T.G. Madeno˘glu, E. Yildir, M. Sa˘glam, M. Yuksel, L. Ballice, Improvement in hydrogen production from hard-shell nut residues by catalytic hydrothermal gasification, J. Supercrit. Fluids 95 (2014) 339–347. [20] R.F. Nascimento, J.C. Marques, B.S.L. Neto, D.D. Keukeleire, D.W. Franco, Qualitative and quantitative high-performance liquid chromatographic analysis of aldehydes in Brazilian sugar cane spirits and other distilled alcoholic beverages, J. Chromatogr. A 782 (1997) 13–23. [21] T. Yoshida, Y. Matsumura, Gasification of cellulose xylan, and lignin mixtures in supercritical water, Ind. Eng. Chem. Res. 40 (2001) 5469–5474. [22] D. Klingler, H. Vogel, Influence of process parameters on the hydrothermal decomposition and oxidation of glucose in sub- and supercritical water, J. Supercrit. Fluids 55 (2010) 259–270. [23] M. Watanabe, Y. Aizawa, T. Iida, C. Levy, T.M. Aida, H. Inomata, Glucose reactions within the heating period and the effect of heating rate on the reactions in hot compressed water, Carbohydr. Res. 340 (2005) 1931–1939. [24] K. Lundquist, L. Ericsson, Low-molecular weight lignin hydrolysis products, Appl. Polym. Symp. 28 (1976) 1393–1407. [25] M. Saisu, T. Sato, M. Watanabe, T. Adschiri, K. Arai, Conversion of lignin with supercritical water-phenol mixtures, Energy Fuels 17 (2003) 922–928.
85
[26] A. Kruse, H. Vogel, Heterogeneous catalysis in supercritical media: 2. Near-critical and supercritical water, Chem. Eng. Technol. 31 (2008) 1241–1245. [27] A. Sinag, A. Kruse, V. Schwarzkopf, Formation and degradation pathways of intermediate products formed during the hydropyrolysis of glucose as a model substance for wet biomass in a tubular reactor, Eng. Life Sci. 3 (2003) 469–473. [28] R. Muangrat, J.A. Onwudili, P.T. Williams, Alkali-promoted hydrothermal gasification of biomass food processing waste: a parametric study, Int. J. Hydrogen Energy 35 (2010) 7405–7415.